1. Field of the Invention
The present invention relates to permanent seismic wave emission systems and more particularly to a permanent seismic wave emission device based on magnetostrictive materials.
2. Description of the Prior Art
Several types of seismic sources for emitting seismic waves in wells are known. These sources are of different types and involve
either an explosion which may be a point or elongated charge with detonating cord wound helically around a rigid mandrel, etc.;
an electric spark which is a breakdown between electrodes in water, a wire explosion under the effect of an electric discharge, etc.,
a vertical mechanical shock of a mass impacting an anvil secured to a packer that produces, on the well wall, a vertical shear for mainly generating S waves,
or a horizontal shock, in a radial direction, of a mass radially driven by hydraulic or electromagnetic a mechanism that contact the well wall at one point.
Seismic waves are also created in wells using controlled vibratory sources of piezoelectric type which are coupled (or not) to the well wall and which emit monofrequencies or codes or frequency modulated signals through a ramp.
These permanently buried seismic sources provide advantages of stability of the source coupling conditions due to the fact that the source is permanently cemented in a dedicated well; insensitivity to the climatic variations of the altered zone when the source is installed below the altered zone; ease of implementation of the source for each acquisition survey; and proximity of the source in relation to the seismic objective to be imaged.
The main drawback of these buried permanent seismic sources is the high installation cost, mainly because a well dedicated to these sources is required. Furthermore, their efficiency is affected by the stiffness of the casing that limits the stress applied to the surrounding medium.
A device for emitting radial elastic waves in a material medium, based on the principle of magnetostriction, is also known. The emission of radial waves is essentially obtained by radially expanding the metallic casing of the well in contact with the material medium under the effect of a magnetic pressure generated by electromagnetic induction with elastic waves being created in the medium under the effect of the radial expansion. The magnetic pressure is obtained by connecting a coil placed in line with the tube to a current generator, a shock generator or a variable frequency generator. The winding pitch of the coil can be constant or variable. The tube can for example be added into a well or a hole, or it can be a tube portion of a cased well. It can also be the lateral wall of a sealed enclosure that is immersed in water for producing acoustic or seismic waves therein.
However, for this type of source, the elongation of the material making up the casing is tens of parts per million. The seismic wave generated is thus very weak and the efficiency of this type of source is therefore not satisfactory enough to be properly used in the industry.